As an example of a conventional method of continuously driving an alternating current motor by using an inverter at the time of an instantaneous power failure, Japanese Patent No. 3201460 (Patent Document 1) discloses the following method: an inverter starts decelerating an alternating motor in response to a power failure detecting signal; a first reduction rate is calculated on the basis of a target value and a detection value of a direct current intermediate voltage such that the direct current intermediate voltage is made constant during the deceleration; a second reduction rate is calculated on the basis of a variation in the direct current intermediate voltage; a value obtained by multiplying the two reduction rates together is controlled in a PI control manner to control a deceleration time; and when the direct current intermediate voltage is equal to a voltage before the power failure is detected or the direct current intermediate voltage rises during the deceleration, the deceleration stops and the alternating current motor is returned to a normal control mode.
Further, as an example of a method of processing a motor at the time of a power failure, JP-A-11-208894 (Patent Document 2) discloses the following method: a lower limit voltage VU1 of a direct current intermediate voltage required for a normal driving mode, a lower allowable voltage VU0 of the direct current intermediate voltage when a lowest voltage of a power supply is input, and a power failure detecting level voltage VU2 which is lower than the lower allowable voltage VU0 and is higher than the lower limit voltage VU1 are set; when a detection value VPN of the direct current intermediate voltage output from a direct current intermediate voltage detecting circuit is lower than the power failure detecting level voltage VU2 during the driving of the alternating current motor, an alternating current motor is decelerated at a reduction rate αd until the direct current intermediate voltage is higher than the lower allowable voltage VU0; and when the direct current intermediate voltage is higher than the lower allowable voltage VU0, the alternating current motor is returned to a normal control mode.
Patent Document 1: Japanese Patent No. 3201460
Patent Document 2: JP-A-11-308894